1. Technical Field
The present invention relates to a micro heater and a micro sensor, more particularly, relates to a micro heater and a micro sensor, wherein an air gap surrounding the heater wire is formed, and the heater wire is formed on a porous substrate.
2. Description of the Related Art
As attention on the environmental issue is gradually increasing in these days, development of a small sensor which can obtain accurate and various information in a short time is required. Especially efforts on miniaturization, accuracy enhancement, and cost reduction of the gas sensors in order to measure related gas concentration easily for a comfortable living space, management of harmful industrial environment, food price, food production process management, and the like, have been progressed.
Nowadays, gas sensors are gradually evolving from the conventional structure comprising sintered ceramics or thick films to the structure comprising Micro Electro Mechanical System (MEMS) by adopting semiconductor process technology.
Reviewing from the aspects of measurement methods, currently the most widely used technique for a gas sensor is to measure the change in the electrical characteristics of the sensing material of the sensor when gas is being absorbed therein. Metal oxide such as SnO2 is commonly used as a sensing material, and it is advantageous in that the measurement method is relatively simple wherein the change in the electrical conductivity is measured according to the gas concentration of the object gas. At this time, the sensing material of the metal oxide is heated to a high temperature, and the variation in the measurement values during the operation thereof becomes more significant, therefore, precise temperature control is essential for a fast and precise measurement of the gas concentration. And, when measuring, residual gases or moistures which had already been absorbed in the sensing material are being forcibly removed by heating with high temperature, and the sensing material is reset to the original state thereof, and then the gas concentration is measured. Therefore, the temperature characteristics of the gas sensor directly affect key measurement factors such as sensitivity, recovery time, response time, and the like of the sensor.
Thus, for an efficient heating, a micro heater type is effective which can heat only the sensing material locally and uniformly. However, when measuring with a micro gas sensor, if large power consumption is needed for temperature control, then a large battery or a large power source is required even though the volume of the sensor and measurement circuit is small, thus, the size of the entire measurement system will be determined by these factors eventually. Therefore, in order to implement a micro gas sensor, a structure of low power consumption must be considered in the first place.
Until this time, when manufacturing most micro gas sensors silicon substrate of a very large heat conductivity is commonly used, therefore, in order to reduce heat loss a suspended structure separated from the substrate is formed by forming an etched pit or a groove inside the sensor structure using a bulk micromachining process, and then micro heater, insulation layer, sensing material, and the like are sequentially formed on this structure, and in such a way, a portion of the heat transfer loss can be reduced. However, in this case, since the manufacturing method is based on a wet etching process utilizing the crystalline directivity of the substrate itself, there is a limitation in miniaturization of the sensor element, furthermore, there has been a problem of compatibility with the standard CMOS semiconductor process due to the physical property of the etchant being used such as potassium hydroxide (KOH) and the like.